[libc++] Adds missing forward_list merge tests.

During the review of D112660 it turned out the tests for
`std::forward_list::merge` are incomplete.

Adds tests for the rvalue reference overloads. The tests are extended to
better test the Effects [forward.list.ops]/25 and Remarks
[forward.list.ops]/27 of the function:
- x is empty after the merge.
- Pointers and references to the moved elements of x now refer to those
  same elements but as members of *this.
- Iterators referring to the moved elements will continue to refer to
  their elements, but they now behave as iterators into *this, not into x.
- The algorithm is stable.

Reviewed By: Quuxplusone, #libc, ldionne

Differential Revision: https://reviews.llvm.org/D113364